[RFC] serial: mxc: get the clock frequency from the used clock for the device

[Sean Anderson]

On 3/17/22 3:14 PM, Heiko Thiery wrote:
> Hi Sean,
> 
> Am Do., 17. März 2022 um 15:38 Uhr schrieb Sean Anderson <seanga2 at gmail.com>:
>>
>> Hi Heiko,
>>
>> On 3/17/22 8:41 AM, Heiko Thiery wrote:
>>> With the clock driver enabled for the imx8mq, it was noticed that the
>>> frequency used to calculate the baud rate is always taken from the root
>>> clock of UART1. This can cause problems if UART1 is not used as console
>>> and the settings are different from UART1. The result is that the console
>>> output is garbage. To do this correctly the UART frequency is taken from
>>> the used device. For the implementations that don't have the igp clock
>>> frequency written or can't return it the old way is tried.
>>>
>>> Signed-off-by: Heiko Thiery <heiko.thiery at gmail.com>
>>> ---
>>>    drivers/serial/serial_mxc.c | 15 +++++++++++++--
>>>    1 file changed, 13 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/serial/serial_mxc.c b/drivers/serial/serial_mxc.c
>>> index e4970a169b..6fdb2b2397 100644
>>> --- a/drivers/serial/serial_mxc.c
>>> +++ b/drivers/serial/serial_mxc.c
>>> @@ -3,6 +3,7 @@
>>>     * (c) 2007 Sascha Hauer <s.hauer at pengutronix.de>
>>>     */
>>>
>>> +#include <clk.h>
>>>    #include <common.h>
>>>    #include <dm.h>
>>>    #include <errno.h>
>>> @@ -266,9 +267,19 @@ __weak struct serial_device *default_serial_console(void)
>>>    int mxc_serial_setbrg(struct udevice *dev, int baudrate)
>>>    {
>>>        struct mxc_serial_plat *plat = dev_get_plat(dev);
>>> -     u32 clk = imx_get_uartclk();
>>> +     u32 rate = 0;
>>> +
>>> +     if (IS_ENABLED(CONFIG_CLK)) {
>>
>> CONFIG_IS_ENABLED?
> 
> This should be correct. The CONFIG_IS_ENABLED is a preprocessor macro
> and the IS_ENABLED can be used in c code. Or do you mean something
> else?

I mean you should be using CONFIG_IS_ENABLED(CLK) so that this code is
correct for both SPL and U-Boot proper. But it is also fine to "try and
fail" (making this check unnecessary).

>>
>> mx6ull at least does not have CONFIG_SPL_CLK enabled.
> 
> I expect that in that case it will fallback to the old behavior ... not?

Yes, if you handle -ENOSYS correctly.

>>
>>> +             struct clk clk;
>>> +             if(!clk_get_by_name(dev, "ipg", &clk))
>>> +                     rate = clk_get_rate(&clk);

You may also need to enable this clock.

>>> +     }
>>> +
>>> +     /* as fallback we try to get the clk rate that way */
>>> +     if (rate == 0)
>>
>> !rate || IS_ERR_VALUE(rate)
>>
>>> +             rate = imx_get_uartclk();
>>>
>>> -     _mxc_serial_setbrg(plat->reg, clk, baudrate, plat->use_dte);
>>> +     _mxc_serial_setbrg(plat->reg, rate, baudrate, plat->use_dte);
>>>
>>>        return 0;
>>>    }
>>>
>> --Sean
>